1. Field of the Invention
The present invention relates to a throttle device for an internal combustion engine which performs an opening degree control of a throttle valve for the internal combustion engine with an electrical actuator and, more specifically, to a throttle device for an internal combustion engine with a mechanism in which an initial opening degree of a throttle valve for the internal combustion engine, when an engine key switch is turned off, is set larger than that of a fully closed position of the throttle valve.
2. Description of Conventional Art
In conventional electric throttle control schemes in which a throttle valve for an internal combustion engine is driven and controlled by an electrical actuator, it was proposed to set larger an initial opening degree of the throttle valve (hereinbelow also called as a throttle initial opening degree), when an engine key switch is turned off, in other words when no current is supplied to a motor for the throttle actuator, than that of fully closed position of the throttle valve which corresponds to an opening degree for a normal idling operation after completing a warming-up operation.
One of reasons of setting the initial opening degree as such is to ensure a necessary air flow rate for combustion in a prior warming-up operation during engine starting, in other words in a cold climate starting. Further, during an idling operation a control is performed in which depending on an advancement of the warming-up operation the opening degree of the throttle valve is gradually restricted from the initial opening degree to that of the fully closed position which corresponds to an opening for a normal idling operation.
Other reasons of setting the initial opening degree as such are to ensure self pulling for permitting a limp home function in case when a throttle control system malfunctions, to ensure a predetermined air flow rate for preventing a possible engine stop and to prevent the throttle valve from adhering to the inner wall of a throttle body by such as adhering material and ice pieces contained in the air.
For example, JP(PCT)-A-2-500677 discloses a return spring serving as a first urging means which urges a throttle valve in its closing direction and a resisting spring serving as a second urging means or a spring used for determining an initial opening degree which urges the throttle valve in its opening direction against the return spring wherein the spring force of the resisting spring of the latter at the position of the throttle initial opening degree is set larger than that of the return spring and during an engine key switch being turned off a free end of the resisting spring is engageably stopped by a stopper at the position of the throttle valve initial opening degree so as to hold the throttle initial opening degree.
Further, JP-A-3-271528 discloses the following structure in which a sleeve serving as a supporting member as well as one of constituting elements for a relief lever is fitted in a boss portion on a side wall of a throttle body where an end of a throttle shaft is supported so as to permit free movement in a rotating direction, the relief lever is urged by a return spring serving as a first urging means in the direction for closing the throttle valve, on one hand, a throttle lever is secured to the throttle shaft and the relief lever is then engaged to the throttle lever by the spring force of the return spring, and during no current being supplied to a motor the throttle valve is moved through the engagement of the relief lever and the throttle lever and through the spring force of the return spring up to a predetermined position where the opening degree of the throttle valve is larger than that at the fully closed position thereof and is stopped there by a stopper and at this predetermined position the throttle lever is urged in the direction of opening the throttle valve by making use of a second urging means to thereby hold a throttle initial opening degree.
Still further, JP-A-4-203219 discloses a structure in which a lever is secured at an end of a throttle shaft so as to cross therewith, a return spring serving as a first urging means applies an urging force to one end of the lever in the direction of closing the throttle valve, a second urging means applies an urging force to the other end of the lever in the direction of opening the throttle valve near a throttle fully closed position, and the urging force of the second urging means is set larger than that of the first urging means when a throttle opening degree is below a predetermined opening degree, in that below the throttle initial opening degree to thereby maintain the throttle initial opening degree.
In these conventional examples, controls of the throttle valve opening degree are performed through drive controls of their motors based on control signals transmitted from their control systems and when a throttle valve opening degree is desired to be reduced less than the throttle initial opening degree, a driving torque by the motor is effected to move the throttle valve in the closing direction against the second urging means.
In these sorts of the initial opening degree setting mechanisms such as disclosed in JP-A-3-271528, since such as the relief lever, the throttle lever being engaged with the relief lever, the first and second urging means and the stopper are disposed at around one end of the throttle shaft, a collective arrangement of these parts is achieved. However, in such conventional structure, the sleeve constituting one element of the relief lever and serving as a supporting member is fitted into the boss portion formed on the side wall of the throttle body, therefore, when the sleeve follows the rotation of the throttle shaft during control of the throttle opening degree, then the sleeve slides around the outer circumference of the boss portion to thereby cause a friction between the sleeve and the boss portion which is required to be reduced as much as possible, because such friction operates as a load with respect to the return spring and the motor drive.
Further, in connection with the collective arrangement of the parts for the throttle initial opening degree setting mechanism, when a so called limp home mechanism is incorporated which, in case when circuits and actuators for throttle control unit in an electrically control throttling system or an electric throttle control system malfunction, permits self pulling by mechanically coupling the acceleration pedal with the throttle valve, a collective and rational arrangement of the parts of the limp home mechanism is also desired.
In this sort of the limp home mechanism, since the throttle shaft is driven and controlled by a motor during a steady state traveling, the acceleration lever for the self-pulling never engages with the throttle lever directly connected to the throttle shaft, when the acceleration pedal is depressed. However, during a traction control such as for preventing slipping the throttle lever is suddenly returned with respect to the acceleration lever and the throttle lever resultantly engages with the acceleration lever to thereby cause a kick-back of applying a return force on the acceleration lever which was one of problems to be solved.
Further, both a throttle shaft torque T1 which is provided by the return spring serving as the first urging means uring the throttle valve in its closing direction and another throttle shaft torque T2 which is provided by the spring used for predetermining the initial opening degree and serving as the second urging means urging the throttle valve in its opening direction are generally designed to satify the following two inequations in order to keep a predetermined margin at the position of the throttle initial opening degree EQU T1&gt;Mf.times.Ge+Vf EQU T2&gt;Mf.times.Ge+Vf
wherein, Mf; friction torque when the motor is standstill, Ge; reduction gear ratio, Vf; necessary torque to be applied on the throttle shaft so as to open the throttle valve.
In JP(PCT)-A-2-500677 and JP-A-4-203219, it is designed that a relationship of T1&lt;T2 stands below a predetermined throttle opening degree near the throttle fully closed position. On the other hand, in JP-A-3-271528, it is designed to permit a relationship of T1.gtoreq.T2, because the spring force of the return spring serving as the first urging means is received by the stopper for the initial opening degree and the shaft torque T2 at the position of the throttle initial opening degree can be set without being restricted by the shaft torque T1. Anyway, the first and the second urging means are designed based on the above two inequations.
Since T1 is a shaft torque which urges the throttle valve in its closing direction which is hereinbelow assumed as positive direction and T2 is a shaft torque which urges the throttle valve in its opening direction which is hereinbelow assumed as negative direction, as explained above, a shaft torque stepped difference T1-(-T2) between the throttle shaft torques due to the first and second urging means is generated which suddenly changes at the reference position of the throttle initial opening degree. Since the larger the shaft torque stepped difference the harder the control of the throttle valve opening degree, it is preferable to minimize the shaft torque stepped difference in order to increase an accuracy of the throttle valve control.